The present invention relates to wireless communication receivers, and particularly relates to demodulating amplitude-modulated signals.
Higher-order modulation constellations represent one mechanism supporting the faster data rates of current and evolving wireless communication systems. For a given symbol rate, more bits in each modulation symbol translates into higher effective data rates, assuming acceptable demodulation performance at the receiver. The High Speed Downlink Packet Access (HSDPA) extension of the WCDMA standards supports higher-level modulation in its higher data rate modulation/coding schemes. For example, the High Speed Downlink Shared Channel (HS-DSCH) uses 16-QAM in one or more of its modulation/coding schemes.
Use of 16 QAM on the HS-DSCH enables higher data rates, but such use complicates data reception. Specifically, an amplitude reference is required at the conventional receiver to detect received symbols and properly scale the soft information for decoding (e.g., by a turbo decoder). For HS-DSCH, and for CDMA-based pilot-and-traffic-channel transmissions in general, the needed amplitude reference represents the relative scaling between the code channel used for estimation (e.g., the pilot channel) and the code channel(s) being demodulated (e.g., the traffic channels).
The needed scale factor g can be written ashtraf=ghpil  Eq. 1where hpil represents the channel response vector as estimated from the received pilot channel signal, htraf represents the channel response vector for the traffic channel, which is unknown, and g is the scale factor. To illustrate the usefulness of the scale factor, consider the Log-Likelihood Ratio (LLR) for bit bj of the ideal QAM demodulator, which is given as
                              LLR          ⁡                      (                          b              j                        )                          =                                            ∑                                                s                  i                                ∈                                                      S                    0                                    ⁡                                      (                    j                    )                                                                        ⁢                                                  ⁢                          exp              ⁢                              {                                  γ                  (                                                            2                      ⁢                                              Re                        (                                                                                                            s                              i                              *                                                        ⁢                            z                                                                                                              w                              H                                                        ⁢                                                          h                              traf                                                                                                      )                                                              -                                                                                                                    s                          i                                                                                            2                                                        )                                }                                                                        ∑                                                s                  i                                ∈                                                      S                    1                                    ⁡                                      (                    j                    )                                                                        ⁢                                                  ⁢                          exp              ⁢                              {                                  γ                  (                                                            2                      ⁢                                              Re                        (                                                                                                            s                              i                              *                                                        ⁢                            z                                                                                                              w                              H                                                        ⁢                                                          h                              traf                                                                                                      )                                                              -                                                                                                                    s                          i                                                                                            2                                                        )                                }                                                                        Eq        .                                  ⁢        2            where z is the symbol estimate made by the receiver, si is a candidate symbol from a normalized scale constellation, γ is the signal-to-noise ratio (SNR), which actually may be calculated as a signal-to-interference-plus-noise ratio (SINR), and w represents a vector of combining weights used to form the estimated symbol z.
With incorporation of the proper scaling and assuming a log-max turbo decoder and the use of the pilot channel for estimating channel coefficients, the log-likelihood ratio becomes
                                          LLR            ⁡                          (                              b                j                            )                                =                      γ            ⁡                          [                                                                    max                                                                  s                        i                                            ∈                                                                        S                          0                                                ⁡                                                  (                          j                          )                                                                                                      ⁢                                      (                                                                  2                        ⁢                                                  Re                          (                                                                                                                    s                                i                                *                                                            ⁢                              z                                                        μ                                                    )                                                                    -                                                                                                                              s                            i                                                                                                    2                                                              )                                                  -                                                      max                                                                  s                        i                                            ∈                                                                        S                          1                                                ⁡                                                  (                          j                          )                                                                                                      ⁢                                      (                                                                  2                        ⁢                                                  Re                          (                                                                                                                    s                                i                                *                                                            ⁢                              z                                                        μ                                                    )                                                                    -                                                                                                                              s                            i                                                                                                    2                                                              )                                                              ]                                      ,                            Eq        .                                  ⁢        3            where the normalization factor μ is defined asμ=wHhtraf  Eq. 4
The normalization factor is used to normalize the symbol estimate z for comparison to the symbols si in a normalized scale modulation constellation. The scale factor commonly is determined as an explicit, additional step, through time estimation of the RMS value of the estimated symbol,√{square root over (|z|2)}≈wHhtraf=μ.  Eq. 5The above estimation represents an explicit computational step that is carried out by the conventional receiver after generation of the estimated symbols z.
In addition to the explicit pilot-to-traffic scaling needed for the symbol estimates, use of the pilot channel in other aspects of receiver operation can be problematic. For example, conventional Generalized RAKE (G-RAKE) receiver processing generates the combining weights w a function of received signal impairment correlations. Considering impairment correlations in the combining weight generation process allows the G-RAKE receiver to cancel colored (correlated) interference across its RAKE fingers, leading to reduced interference in the estimated symbols z.
As a baseline approach, G-RAKE receivers estimate noise correlations for a received communication signal using despread pilot values. Commonly, the impairment correlations are assumed to have zero mean and, as such, they are expressed in terms of a noise correlation matrix determined from the pilot symbols despread from the received communication signal.
Regardless of such details, however, using pilot symbols constrains the number of despread values available for estimating the impairment correlations over any given reception interval. As an example, the Wideband Code Division Multiple Access (W-CDMA) standards provide for Transmission Time Intervals (TTIs) of varying “slot” counts. Generally, one Common Pilot Channel (CPICH) symbol is transmitted per slot. Thus, for the three-slot TTI associated with the High Speed Downlink Shared Channel (HS-DSCH) used in W-CDMA networks to provide high-rate packet data services, relatively few pilot symbols are available for noise correlation estimation per TTI.
The relatively small number of pilot symbols available for correlation estimation can lead to poor noise correlation estimates. Averaging the noise correlation estimations under such circumstances can lead to reduced estimation error, but such smoothing compromises the receiver's ability to respond to fast fading conditions.